FIG. 1 generally illustrates a prior art technique for forming a wide area light sheet or panel using LEDs. A starting substrate 10 may be Mylar or other type of polymer sheet, or even a metal sheet. A conductor layer 12 is then deposited over the substrate 10, such as by printing. The substrate 10 and/or conductor layer 12 is preferably reflective. A reflective film may also be provided on the front or back surface of the substrate 10.
An LED ink is provided, which comprises microscopic vertical LEDs 14 (e.g., 30 microns in diameter) uniformly infused in a solvent. The LEDs are initially formed as metallized semiconductor layers on a carrier wafer. Trenches are photolithographically etched through the semiconductors layers and the metal layers to define the boundaries of each LED. The back surface of the carrier wafer is then thinned until the individual LEDs are separated. The LEDs are then dispersed in the solvent to create the ink.
The LED ink is screen printed over the conductor layer 12. The orientation of the LEDs can be controlled by providing a relatively tall top electrode 16 (e.g., the anode electrode), so that the top electrode 16 orients upward by taking the fluid path of least resistance through the solvent after printing. The LED ink is heated to evaporate the solvent, and the bottom cathode electrode 18 and conductor layer 12 are annealed to create an ohmic cathode connection.
A dielectric 19 is deposited and etched to expose the top electrode 16.
A transparent conductor layer 20 is then printed to contact the top electrodes 16.
Metal bus bars 22 and 24 are then printed and cured to electrically contact the conductor layers 12 and 20 along their edges. A suitable voltage differential applied to the bus bars 22/24 turns on the LEDs 14. Although the microscopic LEDs 14 are randomly distributed, they are fairly uniformly distributed over the area of the flat sheet due to the large number of LEDs printed. There may be millions of LEDs 14 printed on a one square meter substrate 10. The fabrication process may be performed under atmospheric conditions.
The LEDs 14 in the monolayer, within a defined area, are connected in parallel by the conductor layers 12/20 since the LEDs 14 have the same orientation. If many LEDs 14 are connected in parallel, the driving voltage must approximately equal the voltage drop of a single LED 14 and the current is relatively high. The high current flowing laterally through at least the thin transparent conductor layer 20 creates a significant IR drop, since typical transparent conductors may have a conductivity of 1 ohm/square. This results in power loss and heat, lowering the efficiency of the lamp. Making the transparent conductor layer 20 thicker adds cost and increases the light absorption by the layer 20.
Further detail of forming such a light source by printing microscopic vertical LEDs, and controlling their orientation on a substrate, can be found in US application publication US 2012/0164796, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.
It is common to connect discrete LEDs in series by using printed circuit boards and other techniques. By connecting LEDs in series, the driving voltage increases and the driving current is lowered. However, such electrical interconnections are impractical for printed LEDs, since the LEDs are randomly positioned and microscopic. Further, using lateral conductors to connect a layer of LEDs in series uses significant substrate area, creating noticeable dark areas between the LEDs and lowering the brightness-to-area ratio of the lamp.
What is needed is a practical and cost-effective technique for connecting printed LEDs in series while still obtaining a high density of LEDs for a good brightness-to-area ratio.